In some known machine tools such as a numerically controlled (NC) automatic lathe capable of implementing various kinds of automatic lathe-turning machining (hereinafter generically called an automatic lathe), a guide bush is provided in the vicinity of a position where machining is performed with a tool, and the tip of a long rod-like material (hereinafter referred to as a rod material) gripped by a chuck at the tip of a spindle is supported by the guide bush and thus machined (e.g., refer to Japanese Patent Publication Laid-open No. 4-115804).
In the machining of the rod material using the guide bush, a part to be machined at the tip of the rod material is supported so as not to cause deflection during lathe-turning machining, such that there is an advantage that even an elongated product can be continuously and highly accurately machined from the long rod material.
However, in spite of the advantage described above, the automatic lathe equipped with the guide bush has a problem that there is a gap, which is small, between the guide bush and the rod material, and the rod material moves due to this gap, which makes the automatic lathe unsuitable to machine products requiring more accurate machining.
Furthermore, there is a problem that, due to a condition that the dimension of the rod material be longer than at least a distance between a grip position of the chuck at the forward end of the spindle and a machining position in the vicinity of the guide bush, it is not possible to machine a rod material shorter than this distance. Moreover, there is a problem that if a long rod material is machined, a material between the grip position and the machining position results in a remaining material.
Thus, the automatic lathe equipped with the guide bush has a problem of increased equipment cost and machining cost because its use is limited to the machining of the rod materials having a length equal to or longer than a certain length.
On the other hand, another automatic lathe has. been proposed, wherein when highly accurate machining is carried out for a relatively short product or when machining is carried out under a machining condition with a heavy cutting load, a guide bush is suitably removed, thereby making it possible to switch from machining with the guide bush to machining without the guide bush (e.g., refer to Japanese Patent Publication Laid-open No. 9-225703).
However, in the technique described in this document, it is necessary to insert the tip of a spindle into a protection hole placed instead of the guide bush when machining is carried out without the guide bush. Thus, a certain length of the tip of the spindle protrudes from a tool post, but the part protruding from the tool post is not supported. Therefore, the tip of the spindle is in an overhanging state, and its rigidity is reduced. This leads to a problem that the tip of the spindle bends to result in a decreased machining accuracy when machining a relatively short product requiring high machining accuracy which needs to be machined without using the guide bush and when machining is carried out with a heavy cutting load.
Furthermore, a lathe of a type in which a tool post is movable has been proposed, wherein in order to increase an accuracy when machining is carried out with a heavy cutting load, a guide bush having a chuck function is held at the tip of a spindle held on the tool post, and a material feed shaft comprising a chuck is provided within the spindle movably with respect to the spindle (e.g., refer to U.S. Pat. No. 2,750,356).
However, even the technique described in this document can not solve the above-mentioned problem of producing a remaining material. Moreover, there are needed a mechanism to open/close the chuck of the guide bush, and a mechanism to open/close the spindle chuck, so that the configuration of the automatic lathe is complicated and the length of the spindle is increased, leading to another problem that the overall length of a machine is increased.
The present invention has been made in view of the foregoing problems, and is directed to provide an automatic lathe comprising a guide bush which rotates at the same speed as a material. The automatic lathe is capable of easily switching between the machining of the material with the guide bush and the machining of the material without the guide bush. The automatic lathe has a simple and compact configuration in which a spindle is rigidly supported up to its tip or up to the vicinity of the tip to increase mechanical rigidity, such that machining accuracy is not decreased even when the material is machined without using the guide bush. Moreover, the switching operation can be easily performed by a user who is not a person in charge of, for example, a manufacturer of the automatic lathe.